This invention relates generally to an overdrive transmission adapted for use in the driveline of an automotive vehicle. More particularly, it is directed to an overdrive transmission for use between the mainshaft of a conventional transmission and the vehicle propeller shaft. The overdrive transmission operates automatically in the direct drive mode, the direct coast mode and the reverse mode. It is shiftable smoothly into and out of the overdrive mode.
The typical overdrive transmission includes a gear set of some kind which has two operating conditions; one providing a direct, one-to-one ratio and another providing an overdrive ratio. A clutching device shifts between direct and overdrive at appropriate times. U.S. Pat. No. 2,771,795 issued Nov. 27, 1956 discloses an overdrive transmission in which a one-way clutch provides direct drive, and a wrapped spring friction clutch provides direct coat and reverse. The friction clutch must carry high torque in reverse, and may not hold when the vehicle is parked facing downhill. Furthermore, the friction clutch wastes energy by developing frictional drag when the transmission is in overdrive.
U.S. Pat. No. 2,933,944 issued Apr. 26, 1960 discloses an overdrive transmission in which a jaw clutch replaces the function of the wrapped spring friction clutch. The jaw clutch carries full torque in overdrive and in direct coast. Therefore it is difficult to downshift from overdrive to direct when in coast. Furthermore, a momentary torque reversal is required in order to complete this downshift.
U.S. Pat. No. 3,559,508 issued Feb. 2, 1971 discloses an overdrive transmission in which a jaw clutch carries torque in direct coast. However, the transmission can be downshifted from overdrive to direct only when operating in drive, and not when operating in coast.
U.S. Pat. No. 3,631,741 issued Jan. 4, 1972 discloses an overdrive transmission in which a two-way roller clutch provides driect drive and coast. Here the backlash inherent in this type of clutch causes lurching during the shift between drive and coast. Furthermore, a severe shock to the driveline will be developed where the transmission is downshifted from overdrive to direct while operating in coast.
There remains a need in the art for an overdrive transmission which operates automatically to provide direct drive, direct coast and reverse, and which is shiftable smoothly into and out of overdrive under both drive and coast conditions. The overdrive transmission should not require a friction clutch having a large torque capacity, but rather should allow the use of a small capacity clutch in the interest of space conservation and economy. It should not require an external torque reversal or interruption in order to complete the shift. Further, it should have a direct mechanical connection in the driveline in order to insure that the vehicle does not roll when parked.